Approximately 25,000 intracranial aneurysms rupture each year in North America. The primary purpose of treatment for a ruptured intracranial aneurysm is to prevent rebleeding. There are a variety of ways to treat ruptured and non-ruptured aneurysms.
Possibly the most widely known of these procedures is an extravascular approach which involves surgery or microsurgery. This treatment is common with intracranial berry aneurysms. The method comprises clipping the neck of the aneurysm, performing a suture ligation of the neck, or wrapping the entire aneurysm. Each of these procedures is formed by intrusive invasion into the body and performed from the outside of the aneurysm or target site. General anesthesia, craniotomy, brain retraction, and placement of a clip around the neck of the aneurysm are typically required in these surgical procedures. The surgical procedure is often delayed while waiting for the patient to stabilize medically. For this reason, many patients die from the underlying disease or defect prior to the initiation of the procedure.
Another procedure is the extra-intravascular approach. This approach involves surgically exposing or stereotactically reaching an aneurysm with a probe. The wall of the aneurysm is then perforated from the outside and various techniques are used to occlude the interior in order to prevent it from rebleeding. The techniques used to occlude the aneurysm include electrothrombosis, adhesive embolization, hog hair embolization, and ferromagnetic thrombosis. These procedures are discussed in U.S. Pat. No. 5,122,136 to Guglielmi et al., the entirety of which is incorporated herein by reference.
A still further approach, the least invasive, also is described in Guglielmi et al (supra). It is the endovascular approach. In this approach, the interior of the aneurysm is entered by use of a catheter such as those shown in Engelson (Catheter Guidewire), U.S. Pat. No. 4,884,579 and also in Engelson (Catheter for Guidewire Tracking), U.S. Pat. No. 4,739,768. These patents describe devices utilizing guidewires and catheters which allow access to an aneurysm from remote portions of the body. Specifically, by the use of catheters having very flexible distal regions and guidewires which are steerable to the region of the aneurysm, embolic devices which may be delivered through the catheter are an alternative to the extravascular and extra-intravascular approaches.
The endovascular approach typically includes two major steps. The first step involves the introduction of the catheter to the aneurysm site using devices such as shown in the Engelson patents. The second step often involves filling the aneurysm in some fashion or another. For instance, a balloon may be introduced into the aneurysm from the distal portion of the catheter where it is inflated, detached, and left to occlude the aneurysm. In this way, the parent artery is preserved. Balloons are becoming less in favor because of difficulty in introducing the balloon into the aneurysm sac, the possibility of an aneurysm rupture due to overinflation of the balloon within the aneurysm or due to stress placed on the nonspherically shaped aneurysm by the spherical balloon, and the risk associated with traction produced when detaching the balloon.
A highly desirable embolism-forming device that may be introduced into an aneurysm using endovascular placement procedures, is found in U.S. Pat. No. 4,994,069, to Ritchart et al. The device, typically a platinum/tungsten alloy coil having a very small diameter, may be introduced into an aneurysm through a catheter such as those described in Engelson above. These coils are often made of wire having a diameter of 2-6 mils. The coil diameter may be 10-30 mils. These soft, flexible coils may be of any length desirable and appropriate for the site to be occluded. For instance, the coils may be used to fill a berry aneurysm. Within a short period of time after the filling of the aneurysm with the embolic device, a thrombus forms in the aneurysm and is shortly thereafter complemented with a collagenous material which significantly lessens the potential for aneurysm rupture.
Coils such as seen in Ritchart et al. may be delivered to the vasculature site in a variety of ways including, e.g., mechanically detaching them from the delivery device as is shown in U.S. Pat. No. 5,250,071, to Palermo or by electrolytic detachment as is shown in Guglielmi et al. (U.S. Pat. No. 5,122,136), discussed above.
Guglielmi et al. shows an embolism-forming device and procedure for using that device. Specifically, the Guglielmi device fills a vascular cavity (such as an aneurysm) with an embolic device, typically a platinum coil, that has been endovascularly delivered. The coil is then severed from its insertion tool by the application of a small electric current. Desirably, the insertion device involves a guidewire which is attached at its distal end to the embolic device by a sacrificial joint that is electrolytically dissolvable. Guglielmi et al. suggests that when the embolic device is a platinum coil, the platinum coil may be 1-50 cm. or longer as is necessary. Proximal of the embolic coil is a guidewire, often stainless steel in construction. The guidewire is used to push the platinum embolic coil, obviously with great gentleness, into the vascular site to be occluded. The patent shows a variety of ways of linking the embolic coil to the pusher guidewire. For instance, the guidewire is tapered at its distal end and the distal tip of the guidewire is soldered into the proximal end of the embolic coil. Additionally, a stainless steel coil is wrapped coaxially about the distal tapered portion of the guidewire to provide column strength to the guidewire. This coaxial stainless steel wire is joined both to the guidewire and to the embolic coil. Insulation may be used to cover a portion of the strength-providing stainless steel coil. This arrangement provides for two regions which must be electrolytically severed before the embolic coil is severed from the guidewire.
U.S. patent application Ser. No. 5,423,829, describes a variation of the Guglielmi detachable coil using an improved sacrificial link between the guidewire and the coil. The size of the sacrificial link is limited to allow more precise placement of the embolic device and facilitate quick detachment. The focussed electrolysis found at the sacrificial site reduces the overall possibility of occurrence of multiple electrolysis sites and liberation of large particles from those sites.
The circuit involved in the electrolytic coil detachment arrangements discussed above generally includes a power source having its positive terminal coupled to the sacrificial link via a guidewire, for example. More specifically, a positive electric current of approximately 0.01 to 2 milliamps is applied to the guidewire which is coupled to the sacrificial link that is intended to undergo electrolytic disintegration and which initially couples the implant (e.g., the vasoocclusion device) to the guidewire. The negative terminal of the power source is typically coupled to an electrode that is placed over and in contact with the patient's skin. This can be accomplished, for example, with a large skin electrode such as a ground pad or needle. Thus, the current path flowing from the detachment zone (etch site on the sacrificial link) to the ground electrode includes the patient's fluids and tissues. While the impedance to current flow is generally predictable through the delivery member or guidewire (approximately 200 ohms), the additional impedance of the current's return path to the ground electrode is dependent on variable impedances which depend on the blood milieu of the aneurysm, density of coil packing in the aneurysm, conductivity of the aneurysm wall, patient's tissue and return skin electrode, and fluid volume, for example. If, due to the variable impedances, the impedance to current flow through the occlusive device, which preferably is a platinum coil, becomes less than the impedance to current flow from the detachment zone, it is believed that detachment times will increase.
Another return electrode or cathode arrangement is disclosed in U.S. Pat. No. 5,354,295 to Guglielmi et al. In that arrangement, the microcatheter is supplied with an end electrode. More specifically, the electrode extends distally from the microcatheter and is coupled to an electrical conductor or wire disposed along the length of the microcatheter. The wire is ultimately led back to the negative terminal of the power source so that the electrode (ring electrode) is used as the cathode during electrothrombosis instead of an exterior skin electrode.
According to the '295 patent, the electrical currents and electrical current paths which are set up during electrothrombosis formation using the above-described catheter-electrode arrangement are local to the site of application, which allows even smaller currents and voltages to be used to initiate electrothrombosis than in the situation when an exterior skin electrode must be utilized. The electrothrombosic current distributions are also better controlled and localized to the site of the thrombus formation. The possibility of stray thrombus formations occurring at unwanted sites or uncontrolled and possibly unwanted electrical current patterns being established elsewhere in the brain or body is therefore largely avoided.
However, Applicant has found that under certain conditions, this return electrode arrangement may not provide consistent results. For example, since the return electrode or cathode extends distally from the microcatheter and is directly exposed to the endolumenal environment, it can contact tissue in that environment, such as the wall of the aneurysm. It is believed that such contact would increase the resistance in the current return path from the sacrificial link to the return electrode and increase electrolytic detachment times. In addition, multiple coils typically are placed in the aneurysm to increase the packing density therein and enhance thrombosis. Coils previously placed within the aneurysm may contact the electrode as the microcatheter is positioned at the aneurysm opening to deliver a further coil. It is believed that this may result in the current finding a lower impedance return path through portions of contacting coils than through the blood or fluid between the sacrificial link and the electrode. This may preclude the necessary electrolysis for disintegration of the sacrificial link, since optimal electrolysis occurs when the fluid resistance is less than the coil resistance.